1. Field of the Invention
The invention relates to a process for the selective removal of hydrogen sulfide from gaseous mixtures containing both hydrogen sulfide and carbon dioxide.
2. Discussion of the Prior Art
Processes for the removal of acidic gases such as hydrogen sulfide (H.sub.2 S) and carbon dioxide (CO.sub.2) from gaseous mixtures containing the same are well known in the art. In general, these processes involve scrubbing the gaseous mixture with a liquid absorbent in an absorption zone whereby the acidic gases are removed from the gaseous mixture and a loaded absorbent stream is obtained which is passed to a regeneration zone where the absorbent is heated and/or stripped with solvent vapor, e.g., steam, resulting in the release of the acidic gases. The regenerated absorbent is returned into contact with the feed gas mixture in the absorption zone while the evolved acidic gases are passed to a cooler/condenser in which the solvent vapors are condensed and separated from the acidic gases.
While H.sub.2 S removal to low concentrations is required in most commercial applications, there are a number of cases in which it is not necessary or desirable that carbon dioxide be removed from the gaseous mixture. For example, current pipeline specifications for natural gas permit relatively high concentrations of CO.sub.2 in the product gas as compared to H.sub.2 S which is restricted to very low levels. Thus by selectively removing H.sub.2 S from natural gas, it is possible to treat larger quantities of gas for a given volume of liquid absorbent. In addition, a relatively concentrated H.sub.2 S stream is obtained which is more readily processed into useful by-products such as sulfur or sulfuric acid.
Another application where selective H.sub.2 S removal is desirable is in treating Claus plant off-gases, which typically contain H.sub.2 S, S, S0.sub.2 and CO.sub.2, by catalytically hydrogenating the residual sulfur compounds to H.sub.2 S. Such catalytic hydrogenation processes are described, for example in co-assigned U.S. Application Ser. No. 326,916, filed Jan. 26, 1973, and Canadian Pat. No. 916,898 to Beavon. After hydrogenation the reduced off-gases (tail gas) can be suitably contacted with an absorbent solution to remove H.sub.2 S for recycle to the Claus plant. Since the presence of CO.sub.2 in the gas eventually discharged to the atmosphere is not objectionable, it is desirable that the CO.sub.2 removal in the absorption step be minimized to avoid having to recycle large quantities of CO.sub.2 with the H.sub.2 S which would entail the use of larger blowers, etc. and lead to undesirable accumulations of CO.sub.2 in the system. Thus it is vitally important that H.sub.2 S be selectively removed from the hydrogenated tail gas.
Still another application in which selective H.sub.2 S removal is desirable is in treating acid gas feed streams to a Claus plant (to concentrate the H.sub.2 S), thereby facilitating its conversion to elemental sulfur in the Claus plant. Other instances where selective H.sub.2 S removal is desirable is in the manufacture of dry ice and catalytic processes where a specific amount of CO.sub.2, but no H.sub.2 S is required.
A number of processes have been proposed to improve the selectivity of H.sub.2 S relative to CO.sub.2 in response to the aforementioned needs. Such processes by-and-large are based on either the known difference in the absorption velocity between H.sub.2 S and CO.sub.2, the choice of an absorbent known to have selective adsorption properties, or on the use of an absorbent liquid which is partially saturated with carbon dioxide thereby reducing its capacity to absorb further amounts of CO.sub.2 and hence increase its selectivity for H.sub.2 S. An example of this latter type of process is that described in U.S. Pat. No. 3,266,866 to Bally et al. In the patented process H.sub.2 S and CO.sub.2 are selectively absorbed from a gaseous mixture in an aqueous absorbent solution of a dipropanolamine which is subsequently only partially regenerated so that from 0.02-0.5 moles of CO.sub.2 per mole of dipropanolamine remain in the regenerated absorbent in the form of a carbamate. The use of the partially regenerated solvent in the absorption step was found to increase the selectivity of the absorbent for H.sub.2 S relative to CO.sub.2.
In processes for selectively removing H.sub.2 S based on the difference in the absorption velocity of H.sub.2 S relative to CO.sub.2, H.sub.2 S selectivity is generally improved by reducing the contact time between the absorbent and the H.sub.2 S and CO.sub.2 -containing gaseous mixture, e.g., by increasing gas velocity or reducing the number of contacting stages. While the use of shorter contact times generally enhances H.sub.2 S selectivity, such measures adversely affect the total amount of H.sub.2 S removed and the use of very short contact times can result in unacceptably high H.sub.2 S concentrations in the treated gas. Moreover, the flexibility of processes utilizing short contact times is severely limited because reduction of the acid gas load in the incoming gas stream will increase the contact time with a consequent decrease in selectivity, while an above-normal acid gas load will result in insufficient acid gas removal. The use of high gas velocities to reduce contact time also gives rise to entrainment and foaming problems. The present invention provides a method of achieving very favorable H.sub.2 S selectivity over a wide range of feed rates which is not dependent on high superficial gas velocities, but instead employs a unique combination of processing steps as hereinafter described.